Composite materials are increasingly being applied to aircraft, spacecraft, large space precision optics and various structural components. Reliable quantitative inspection methods can be used to determine the integrity and serviceability of composite structures. The elastic stiffness constants of composites are key contributors to the mechanical behavior and dimensional stability of the related structures. It is therefore important to determine these characteristics accurately.
Destructive tests are often used to determine the elastic properties of the material. These tests are expensive and can only be performed on representative samples, since the samples are eventually destroyed. On the other hand, nondestructive evaluation methods could be used to test each structure itself, rather than only testing a representative sample. Nondestructive evaluation can also be used to determine the status of an aging structure without removing it from service.
Attempts at nondestructive evaluation for material characterization of composites has so far met with limited success. The conventional pulse-echo and through-transmission tests are capable of yielding only one of the five stiffness constants of composites (transversely isotropic material behavior).
The leaky Lamb wave (LLW) technique, pioneered by an inventor of this system, Yoseph Bar-Cohen, uses guided waves which propagate in parallel to the surface of the laminate. This has been shown to yield all the matrix-dominated constants. These constants are indicative of the quality of the material once the correct fibers are chosen. Currently, there is no practical nondestructive method of measuring the matrix-dominated properties. The existing leaky Lamb wave (LLW) test capability has also been slow, e.g. requiring about half an hour for each point.
The LLW data acquisition process involves the acquisition of the reflected wave spectra at various angles of incidence. The amplitude is measured individually for signals in a preselected frequency range. Once this stage is complete, the minima, representing the plate wave modes, that appear on the reflected spectra for each given angle of incidence, are identified. These modes are recorded for the specific angle of incidence and converted to a phase velocity using Snell's law. The process of mode determination is continued for the range of incidence angles that is usually from 12.5.degree. to 50.degree. for graphite/epoxy composite material but may be different for other materials. The curve that is produced is known as the characteristic dispersion curve.
The dispersion curve represents the plate wave modes for the given direction with the fibers. It is useful to measure the dispersion curves for the 0.degree., 45.degree. and 90.degree. polar angles, measured with the first layer of the laminate, as a means of characterizing the laminate.
Once the dispersion data is available, an inversion technique is applied to determine the elastic stiffness constants. The method of inversion, is known in the art and described in Y. Bar-Cohen, A. K. Mal and S. -S. Lih, "NDE of Composite Materials Using Ultrasonic Oblique Insonification," Materials Evaluation, Vol. 51, No. 11, (November 1993) 1285-1296). It has allowed determination of the properties based on single layer data.
Another limitation occurs when testing multi-layered composites because of the large number of associated variables including each layer thickness, density and the presence of a rich epoxy layer at the interfaces.
Setup of the LLW scanner and its control is also complex and requires highly skilled operators. The data acquisition is slow for practical use--about 30 minutes per point. The inversion algorithm has been complex and required the characterization of many variables that are associated with the individual layers of composites.
Thus, while the LLW technique is known, basic problems with the existing capability have restricted its practical application.